Abstract The cause of the ‘wasting disease’ epidemic that struck Zostera marina L. in the early 1930s still remains unresolved. Many researchers have pointed out that some adverse environmental circumstances, prevailing in those years, may have resulted in a weakening of eelgrass, making it more susceptible to Labyrinthula zosterae Porter & Muehlstein, the biological agent of the disease. In this paper, the influence of light intensity, temperature and salinity on the production of phenolic compounds in eelgrass shoots was determined. These compounds are well known to play a role in the chemical defence of a plant against invading microorganisms. Plants grown under a high light intensity showed higher levels of phenolic compounds than those grown under a low light intensity. An increase of the water temperature led to a decrease in phenolic content, salinity showed no influence on the phenolic content. However, infection with Labyrinthula itself proves also to have pronounced effects on the production of phenolic compounds.